1. Technology Field
The present invention generally relates to x-ray generating devices. In particular, the present invention relates to features for implementation in a cathode of an x-ray tube, for example, that prevents contamination or damage to a filament during high temperature operation.
2. The Related Technology
X-ray producing devices, such as x-ray tubes, are extremely valuable tools that are used in a wide variety of applications, both industrial and medical. For example, such equipment is commonly employed in areas such as medical diagnostic examination and therapeutic radiology, semiconductor manufacture and fabrication, and materials analysis.
Regardless of the applications in which they are employed, x-ray tubes operate in similar fashion. In general, x-rays are produced when electrons are emitted, accelerated, and then impinged upon a material of a particular composition. This process typically takes place within an evacuated enclosure of the x-ray tube. Disposed within the evacuated enclosure is a cathode, or electron source, and an anode oriented to receive electrons emitted by the cathode. The anode can be stationary within the tube, or can be in the form of a rotating annular disk that is mounted to a rotor shaft which, in turn, is rotatably supported by a bearing assembly. The evacuated enclosure is typically contained within an outer housing, which also serves as a reservoir for a coolant, such as dielectric oil, that serves both to cool the x-ray tube and to provide electrical isolation between the tube and the outer housing.
In operation, an electric current is supplied to a filament portion of the cathode, which causes a cloud of electrons to be emitted via a process known as thermionic emission. A high voltage potential is placed between the cathode and anode to cause the cloud of electrons to form a stream and accelerate toward a focal spot disposed on a target surface of the anode. Upon striking the target surface, some of the kinetic energy of the electrons is released in the form of electromagnetic radiation of very high frequency, i.e., x-rays. The specific frequency of the x-rays produced depends in large part on the type of material used to form the anode target surface. Target surface materials with high atomic numbers (“Z numbers”) are typically employed. The target surface of the anode is oriented so that the x-rays are emitted as a beam through windows defined in the evacuated enclosure and the outer housing. The emitted x-ray beam is then directed toward an x-ray subject, such as a medical patient, so as to produce an x-ray image.
In order to produce as focused an x-ray beam as possible, it is generally preferred to first shape or focus the stream of electrons emitted from the cathode filament. Such control of electron emission at the cathode in turn results in precise electron impact at the desired location on the anode target surface for desirably focused x-ray emission. Similarly, electron stream shaping by the cathode head prevents “wings,” which are streams of off-focus electrons that serve no purpose other than the reduce the clarity of the resulting x-ray image.
As such, cathodes used in x-ray tubes and other filament-containing devices typically include a head portion that houses the filament. The cathode head can be shaped in order to desirably focus the electrons that are produced by the filament, as mentioned above. Often, the filament is positioned in one or more slots or similar structures that are defined in the cathode head for electron focusing. Further, a close tolerance often exists between the filament and the head surface defining the slot structure, as it has been recognized that minimizing the spacing between the filament and surfaces of the cathode head enables the electron stream to be shaped off-focus wings to be minimized with relatively lower cathode control voltages than what would otherwise be possible.
Unfortunately, the placement of the filament in close proximity to portions of the cathode head, such as slot sides or other similar features, undesirably raises the risk of inadvertent contact of the filament with the cathode head surface during operation of the cathode-containing device, such as an x-ray tube. In detail, during tube operation the filament is electrically energized at a high temperature in order to produce electrons by thermionic emission. At such times, inadvertent contact between the filament and the proximate cathode head surface may occur. Such contact may be precipitated by a transient event, such as mechanical shock to the cathode, a relative voltage spike, or some other occurrence.
Should undesired contact between the filament and cathode head structure occur, damage to the filament may result. In particular, the filament is typically composed of a high melting point, refractory material such as tungsten in order to withstand the temperatures necessary for thermionic emission to be achieved. Cathode heads, in contrast, are often composed of materials that are selected for high voltage compatibility and machinability. Examples of such materials include nickel and nickel alloys, steel, stainless steel, iron and iron alloys, and copper. These materials have melting points lower than that of tungsten. As such, when the hot filament inadvertently contacts the cathode head, it can fuse to the cathode head surface, thus electrically shorting the filament and rendering the cathode unusable.
In other known cathode head configurations, contact between the filament and the cathode head surface is not necessary for damage to nonetheless occur to the filament. For instance, heat emitted from the filament during operation is absorbed by portions of the head structure proximate to the filament. If the proximate head structure is composed of a lower melting point material such as nickel, evaporation of nickel from the head will occur. The nickel evaporate can then redeposit on the filament surface, thereby contaminating the filament and reducing its performance. This filament contamination effect can also occur when the filament touches the head surface but fails to permanently weld to it.
The above-described challenges can be exacerbated in cathode heads that employ “gridding,” a technique used to further control electron emission from cathode by selectively varying the relative electric potential between the filament and the head structure. Unfortunately, however, gridding can often increase relative electrical attraction between the filament and the head structure, thereby increasing chances for undesirable filament contact with the head surface.
Previous attempts to mitigate the above-described challenges have met with only limited success. For instance, cathode head designs have been altered to increase the filament-to-head surface spacing in order to reduce the likelihood of filament-to-head surface contact. But this unfortunately requires that a relatively greater amount of voltage be used to control the filament electron stream during cathode operation.
In light of the above discussion, a need currently exists for filament and cathode assemblies that resolve the challenges described above. In particular, there is a need for a cathode assembly suitable for use in x-ray and other cathode-containing devices that prevents damage to or destruction of a filament from structures proximate thereto during device operation. Any solution should be suitable for filaments employed in stationary and rotary anode x-ray tubes, as well as any devices where unintentional welding or contamination of high temperature filaments is a risk.